LSIII is a long neurotoxin from the venom of the blue green sea snake Laticauda sernifasciata. This sixty-six residue peptide binds to the nicotinic acetylcholine receptor in the postsynaptic membranes of nerve and skeletal muscle with high selectivity and affinity, blocking neuromuscular response and inducing flaccid paralysis. We have recently determined the threedimensional structure of LSHI in solution using NMR. The main elements of secondary structure are a three stranded anti-parallel b-sheet and three finger-like loops protruding from a globular core, consistent with previously determined structures of other long-neurotoxins. The end of the most prominent loop is involved in receptor binding and is disordered relative to the rest of the molecule. We observed that this solvent exposed loop has local order and our results suggest that this region moves as a semi-rigid body, which may play an important role in receptor binding. We have begun to measure the heteronuclear relaxation parameters of LSIII in order to characterize its dynamical behavior. Since isotopic labeling of LSIH is not practical, these experiments are being conducted using 1 3 C at natural abundance. We have completed our measurements of R1 and the precision of exponential fits to the experimental data is on the order of 10%. Preliminary analysis of R2 data indicates that these rates can be determined with similiar precision. Our immediate plans include measuring the heteronuclear NOE and longitudinal two-spin order relaxation rate. With this data in hand we will be in a position to begin to analyze the data in terms of molecular dynamics.